The present invention relates to a radio receiver, and, more particularly, to a radio receiver which has an automatic station-seeking device, for use with an automotive vehicle.
Generally, a radio receiver for an automotive vehicle is provided with a switch which is provided at the output stage of an antenna so as to have a simple circuit structure and which switches the receiving sensitivity in order to avoid interference from nearby stations.
On the other hand, in a radio receiver having an automatic station-seeking device, the station frequency sought is locked by reception of the broadcasting signal. If the reception sensitivity which is used for locking the station frequency is constant when the signal strength is relatively strong, a relatively weak broadcasting signal can be sought. It is very often desired, however, to receive only a relatively strong broadcasting signal in order to get good reception. FIG. 1 shows a typical conventional system for a radio receiver for an automotive vehicle, in which in order to allow the sensitivity of locking the station frequency to be decreased, a change-over switch SW, provided between an antenna 1 and a tuning circuit 2, is switched to the side of an attenuating resistor Rz thereby decreasing the reception sensitivity.
That is, if the switch SW is switched to the side of the resistor Rz, the broadcasting signal, which is reproduced from a speaker 8 through a tuning section consisting of the tuning circuit 2 and a mixer 3, an intermediate frequency amplifier 4, an amplitude limiter 5, a detector 6 and an audio-amplifier 7, is decreased in sensitivity. Simultaneously, what is called a station locking signal which is taken from the input side of the intermediate frequency amplifier 4 to stop sweeping of the automatic tuning circuit 10 in an automatic station-seeking device 9 is also decreased in sensitivity. As a result, if automatic tuning is started with a relatively strong signal, with the frequency sweep control of a phase-locked oscillator 12, by pressing down an automatic tuning key 11, the receiver will not lock on to a station frequency of which the signal received is insufficient to reproduce well, but will only lock on to a received tuned signal which is sufficiently strong to reproduce well. A memory 13 in the automatic station-seeking device 9 stores the frequency information of broadcast signals and directly sets the frequency information in the phase-locked oscillator 12 by a read operation when it is required to find a station.
When during driving of an automotive vehicle, a broadcast is being received using automatic tuning with the receiver sensitivity decreased by operating the switch SW so that the sensitivity of locking on to a station frequency is lowered, good reception is possible so long as the signal strength does not fluctuate. However, if the signal is attenuated, as for example by obstructions such as large buildings, the reproduced output of the received signal is greatly lowered. This attenuation in combination with the lowering of the reception sensitivity can seriously impair the signal to noise ratio.